The present invention relates to the manufacture of electronic devices, and more particularly to the formation of masks employed to fabricate integrated circuits having small features sizes.
As the integrated circuit (IC) manufacturing industry moves toward smaller device dimensions, the resolution enhancement techniques (RET) for optical lithography such as off-axis illumination (OAI), optical proximity correction (OPC) and phase-shifting mask (PSM) have been implemented in conjunction with reducing the wavelength of optical exposure. Lithography at shorter wavelengths imposes a particular challenge for PSM fabrication because the inherent functions of PSMs require optical and physical property control. Thus while OAI is applied at the optical relay system level and OPC often requires only binary transmission, PSMs must generally accurately modify both transmission and phase functions of incident light at the device level. At present, implementation of phase-shifting to fabricate small features consumes much of the cost of manufacturing masks employed to fabricate ICs having present-day critical dimensions.
There are various types of standard PSMs, including alternating aperture PSMs (AAPSMs) and embedded-attenuating PSMs (EAPSMs).
FIG. 1 shows a simplified cross-sectional view of an AAPSM with phase shifting features defined on a UV transparent substrate. AAPSM 1 with phase shifting features is defined on a UV transparent substrate 11 with Cr coating 12. Openings 13 and 14 with thickness difference d such that phase difference between light transmitted through un-shifted opening 15 and phase-shifted opening 16 is 180xc2x0. xe2x80x9cnxe2x80x9d is the refractive index of the substrate 11.
FIG. 2 shows a simplified cross-sectional view of an EAPSM with partially transmitting film. EAPSM 2 has partially transmitting film 23 that shifts the phase of transmitted light by 180xc2x0. EAPSM 2 is built on a UV transparent substrate 21, which is coated with molybdenum silicon-oxynitride (MoSixOyNz) complex 23 and over-coated with a chromium (Cr) layer 22. MoSixOyNz layer 23 attenuates and phase-shifts incident beam 25 relative to the unaltered beam 26.
Fabrication of EAPSMs often requires that mask blanks be coated with a partially absorbing film that shifts the phase of the optical wavefront incident by 180xc2x0 with respect to the coincident wavefront that transmits through the part of the mask not covered by the film (see FIG. 2). This phase difference:
xcex94xcfx86=2xcfx80[n(xcex)xe2x88x921]d/xcex, where:
xcex94xcfx86=phase difference;
n(xcex)=refractive index of the film at a given stepper wavelength xcex; and
d=film thickness,
causes the adjacent wavefronts to destructively interfere with each other. In addition to altering the phase, amplitudes of the two wavefronts near the edge of critical features should be matched with appropriate film absorption in order to achieve desirable destructive interference.
Transmission by the partially transmitting film is given by
T=TO exp[xe2x88x924]k(xcex)d/xcex], where:
TO=initial value of transmission
k(xcex)=the imaginary component of the refractive index at a particular wavelength xcex; and
d=film thickness,
with d determined by the 180xc2x0 phase difference requirement. For a given wavelength, one should control the imaginary component k(xcex), of the refractive index of the film material by controlling its stoichiometry and composition. This task is complicated by the fact that any change in k(xcex) will modify the real component of refractive index n(xcex), as the two quantities are mathematically linked by the Kramers-Krxc3x6nig equation. Consequently, utilizing a single film layer solution will in general not readily lead to arbitrary control over both phase and amplitude.
FIG. 3 shows a cross-sectional view of a mask blank 3 comprising a layer of MoSixOyNz 32 and Cr 33 deposited on UV-grade quartz substrate 31. Mask blanks comprising a single layer of MoSixOyNz have been used for fabricating 248 nm EAPSMs. However, the etch selectivity of MoSixOyNz relative to substrate 31 and the optical properties of mask blank 3 may not be sufficient for applications at or below 193 nm wavelength.
FIGS. 4A-4I show simplified cross-sectional views of multiple lithography and processing steps for fabricating AAPSMs. In FIGS. 4A-4I, multiple lithography and processing steps are utilized in fabricating AAPSMs using Cr coated mask blanks. Mask blank 4 comprises an electron-beam or optical-beam sensitive resist 43 coated on top of a Cr coating 42 on a UV transparent substrate 41. The blanks are subjected to lithography processes 44 and 45 (which may be optical or electron-beam in nature) in combination with wet and dry etching steps to define phase-shifting features.
FIGS. 5A-5I show simplified cross-sectional views of multiple lithography and processing steps for fabricating EAPSMs. In FIGS. 5A-5I multiple lithography and processing steps are utilized in fabricating EAPSMs using MoSixOyNz coated mask blank 5. UV transparent substrate 51 is coated with MoSixOyNz layer 52, Cr layer 53 and then a layer of electron-beam or optical-beam sensitive resist 54. The blank is then subjected to lithography processes 55 and 56 (which may be either electron-beam or optical-beam) with wet and dry etching steps to define embedded-attenuating phase-shifting features.
As illustrated in FIGS. 4A-4I for formation of AAPSMs and in FIGS. 5A-5I for formation of EAPSMs, PSM fabrication methods typically require about eight processing steps, as well as two separate electron/optical beam lithography steps with intervening processing. Each process step exhibits a certain defect rate, and multiple writing with in-between processing generally requires careful wafer handling that can lead to yield problems.
Therefore, there is a need in the art for simple and economical methods and structures for manufacturing PSMs.
The present invention relates to the use of multilayer film stacks and gray scale processing methods to fabricate phase-shifting masks (PSMs) utilized in lithography. Desired optical transmission and phase-shifting functions of the mask are achieved by controlling the optical properties and thickness of constituent film layers. By substantially separating the phase shift and attenuation functions between different film layers, the phase shift mask of embodiments of the present invention can be tuned for optimal performance at various wavelengths more precisely than conventional masks employing a single layer to control both attenuation and phase shifting.
Structures and methods in accordance with the present invention may exploit etch selectivity afforded by the use of appropriate materials in the film stack to obtain improved yields and reduced processing costs for fabrication of PSMs. Careful selection of the materials comprising the films of the mask blanks ensures that when a first layer is patterned by dry etching using a particular chemical gas, the film underlying the first layer will be etched very slowly by that chemical gas. In this manner, the etching system utilized in mask fabrication exhibits high etch selectivity, with each film of the stack behaving as an etch stop for etching of the overlying film. This etch selective property thus overcomes problems in non-uniform etching commonly associated with dry etching systems. The order of formation of film materials comprising the photomask blanks is thus an important aspect of certain embodiments of the present invention.
Processing methods in accordance with embodiments of the present invention may also exploit multi-level electron beam writing techniques to obtain improved yields and reduced processing costs for fabrication of PSMs. When such gray scale processing methods are applied to multilayer mask blanks in accordance with embodiments of the present invention in order to fabricate PSMs, fewer processing steps are involved, handling of the mask blanks is reduced, PSM fabrication yields are improved, and the cycle or turn-around time is shortened.
By combining the etch selectivity of the multilayer materials and multi-level resist lithography and processing approaches in accordance with particular embodiments of the present invention, it is also possible to produce a single integrated PSM (iPSM) having various combinations of RET features including AAPSM, EAPSM, and OPC.
It is thus one object of the present invention to describe embodiments of multilayer structures that can be utilized to produce PSMs with high etch selectivity, gray-scale electron beam lithography and associated processing steps that simplify PSM fabrication, and iPSM fabrication technology. It is another object of the present invention to describe embodiments by which gray scale electron-beam lithography and associated dry and/or wet processing steps can be employed to simplify the PSM fabrication process using currently available PSM mask blank structures.
One embodiment of a mask blank structure in accordance with the present invention comprises an optically transmissive substrate comprising one of quartz and glass. An etch stop layer overlies the optically transmissive substrate, the etch stop layer comprising a diamond-like carbon (DLC) film. A phase-shift layer overlies the DLC etch stop layer and shifts a phase of transmitted light; the phase shift layer susceptible to a first etching process selective against the DLC etch stop layer, the DLC etch stop layer susceptible to a second etching process selective against the optically transmissive substrate.
Another embodiment of a mask blank structure in accordance with the present invention comprises an optically transmissive substrate selected from the group consisting of glass, quartz, fluorinated glass, and fluorinated quartz. A tuned absorption layer comprising diamond-like carbon (DLC) overlies the optically transmissive substrate layer, the tuned absorption layer altering an amplitude of transmitted light. A phase-compensation-layer overlies the tuned absorption layer, the phase-compensation layer shifting a phase of transmitted light.
An embodiment of a process for forming a phase shift mask comprises providing an optically transmissive substrate, forming a phase-shift layer over the optically transmissive substrate layer, and forming an optically opaque layer over the phase-shift layer. The optically opaque layer is removed selective to the underlying phase-shift layer, and then the phase-shift layer is removed selective to the underlying substrate.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached FIGS.